U.S. Pat. Nos. 4,859,858 and 4,859,859 were issued to Knodle et al. on 22 August 1989; and U.S. Pat. No. 5,153,436 was issued to Apperson et al. on 6 October 1992. These three patents disclose analyzers for outputting a signal indicative of the concentration of a designated gas in a sample being monitored by the apparatus.
The gas analyzers disclosed in the '858, '859, and '436 patents are of the non-dispersive type. They operate on the premise that the concentration of a designated gas can be measured by: (1) passing a beam of infrared radiation through the gas, and (2) then ascertaining the attenuated level of the energy in a narrow band absorbable by the designated gas. This is done with a detector capable of generating a concentration proportional electrical output signal.
One important application of the invention at the present time is monitoring the level of carbon dioxide in the breath of a medical patient. This is typically done during a surgical procedure as an indication to the anesthesiologist of the patient's condition, for example. As the patient's wellbeing, and even his life, is at stake, it is of paramount importance that the carbon dioxide concentration be measured with great accuracy.
In a typical instrument or system employing non-dispersive infrared radiation to measure gas concentration, including those disclosed in the '858, '859, and '436 patents, the infrared radiation is emitted from a source and focused into a beam by a mirror. The beam is propagated through a sample of the gases being analyzed. After passing through the body of gases, the beam of infrared radiation passes through a filter. That filter reflects all of the radiation except for that in a narrow band centered on a frequency which is absorbed by the gas of concern. This narrow band of radiation is transmitted to a detector which produces an electrical output signal proportional in magnitude to the magnitude of the infrared radiation impinging upon it. Thus, the radiation in the band passed by the filter is attenuated to an extent which is proportional to the concentration of the designated gas. The strength of the signal generated by the detector is consequently inversely proportional to the concentration of the designated gas and can be inverted to provide a signal indicative of that concentration.
In a typical medical application of the gas analyzers just described, a cuvette is employed to sample a patient's gas exchange via a nasal cannula or to connect an endotracheal tube to the plumbing of a mechanical ventilator. The cuvette confines expired and inspired gases to a specific flow path; and it furnishes an optical path between an infrared radiation emitter and an infrared radiation detector unit, both components of a transducer which can be detachably coupled to the cuvette.
A typical cuvette is molded from an appropriate polymer, and it has a passage defining the flow path for the gases being monitored. The optical path traverses the flow path with apertures in the wall of the cuvette and aligned along and on opposite sides of the flow passage allowing the beam of infrared radiation to enter the cuvette; traverse the gases in the flow passage; and, after being attenuated, exit from the cuvette to the filter and radiation detector. Transmissive sapphire windows in the apertures confine the gases to the cuvette flow passage and keep out foreign matter while minimizing the loss of infrared energy as the beam enters and exits from the cuvette.
Sapphire is a relatively expensive material. Consequently, cuvettes of the character just described are invariably cleaned, sterilized, and reused. The cleaning and sterilization of a cuvette is time-consuming and inconvenient; and the reuse of a cuvette may be perceived as posing a significant risk, especially if the cuvette was previously employed in monitoring a patient suffering from an infectious disease. Another disadvantage of using sapphire windows is that adhesive bonding is the only viable technique for mounting the windows to the cuvette. This technique is slow and expensive, and care must be taken that the windows are accurately positioned.
Efforts have been made to reduce the cost of cuvettes by replacing the sapphire cuvette windows with windows fabricated from a variety of polymers. These efforts have heretofore been unsuccessful.
One, if not the major, problem encountered in replacing sapphire cuvette windows with windows fabricated from a polymer is that of establishing and maintaining a precise optical path length through the sample being analyzed. This is attributable to such factors as a lack of dimensional stability in the polymeric material, the inability to eliminate wrinkles, and the lack of a system for retaining the windows at precise locations along the optical path.
One proposal for solving this problem is made in U.S. Pat. No. 5,067,492 issued 26 November 1991 to Yelderman et al. The patented approach is to squeeze the cuvette between two housing segments of the transducer with which it is used in the course of assembling the cuvette to the transducer. If the same transducer is employed and if its housing is dimensionally stable, this will in theory ensure that the distance between the two cuvette windows is the same each time the same cuvette is used.
This solution has major drawbacks. Squeezing the cuvette is apt to wrinkle or otherwise distort the perhaps initially not distortion-free plastic windows; and this may affect the transmittance of the windows enough to cause a significant error in the concentration of the gas being monitored. Furthermore, in the Yelderman et al. design, the windows are spaced inwardly from the flow passage-associated ends of the optical path apertures. This leaves cavities communicating with the flow passage in which unwanted debris can collect; and these crevices can adversely affect the flow of gases through the cuvette. Also, the structure employed to position and retain the plastic windows in the body of the cuvette is important; and Yelderman et al. contains only the sketchiest of suggestions of how this might be accomplished, let alone a description of a window retaining system that would minimize wrinkles and other distortions and accurately hold the windows in place, particularly considering the squeezing of the cuvette needed to assemble the cuvette to its adapter. Another problem with the Yelderman et al. hardware is that of assembling the cuvette to the adapter because of the interference fit between these two components.